docs: def num_taxonomies_by_site(self):
          """
          :returns: an array with the number of assets per each site
          """
          dic = general.group_array(self.array, 'site_id')
          num_taxonomies = numpy.zeros(self.tot_sites, U32)
          for sid, arr in dic.items():
              num_taxonomies[sid] = len(numpy.unique(arr['taxonomy']))
          return num_taxonomies